Several types of optical communication devices comprise optical waveguides, optical modulators, and optical switching structures made of electro-optic material. A substrate optical waveguide comprises a lower cladding layer formed on the substrate, a core layer having a higher index of refraction formed over the lower cladding layer, and usually an upper cladding layer formed over the core layer. An optical modulator, or an optical switching structure, may be formed in line with the optical waveguide by forming a body of electro-optic (E/O) material on the same level as the core material, with the electro-optic material usually sandwiched between upper and lower cladding layers. Two electrodes are formed on opposing surfaces of the body E/O material, and are used to apply an electric field to the E/O body. The electric field changes selected optical properties (e.g., refractive index, polarization) of the E/O material. The changes in optical properties may be used to achieve various types of modulating, switching, and filtering functions.
A coefficient may be used to relate the change in the optical property of the material with respect to the applied electric field (i.e., the applied voltage divided by the dimension of the material along which the voltage acts). Electro-optic materials are usually crystalline or highly ordered materials (as in the case of polymers). In both cases, the value of the electro-optic coefficient usually depends upon the direction of the electric field relative to the orientation of the material's crystal or highly-ordered structure. Because of this, the electro-optic property is usually specified as a matrix of coefficient values, each of which is measured along a different axis of the material's crystal or ordered structure. This matrix is often called the tensor matrix of the material's property.
In electro-optic devices used in large systems integrated on substrate carriers, the E/O material usually comprises an inorganic single crystalline material, such as lithium niobate, which is difficult to grow and pattern. Single crystalline materials cannot be easily formed on substrate carriers, and must be grown on top of a base crystalline substrate in order to cause the material to form a crystalline structure that follows that of the substrate. Additionally, lithium niobate has a relatively low responses to the applied electric field compared to other inorganic crystalline materials, such as lanthanum-modified lead zirconium titanate (PLZT). However, conventionally grown PLZT layers have relatively high optical losses in the 1550-nm wavelength band, which is a common wavelength for optical communications. The optical loss generally exceeds 2 dB per millimeter of distance traveled through the PLZT layer. Also, the film thickness of the conventionally grown PLZT is so thin that special efforts are need to couple light into the film. In addition, the conventional deposition method usually cannot provide a high quality PLZT film on a large scale substrate. This inability limits the potential for mass production of PLZT films. Thus, the high optical loss value and these geometrical factors limit the practical use of PLZT material in integrated optics applications.